This application claims the priority of German patent document no. 197 06 858.8, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for determining a rate of rotation by means of a fiber-optic gyro.
The fiber-optic gyro according to the invention has a 3.times.3-coupler with a first channel which is optically connected on one side with a light source and on the other side with a first photo element. The two other channels of the 3.times.3-coupler are optically connected on one side with respective ends of a fiber coil and on the other side with a second or third photo element respectively. A depolarizer is arranged at least between the light source and the 3.times.3-coupler or at one end of the fiber coil and/or the fiber coil having a polarization-maintaining fiber.
U.S. Pat. No. 4,440,498, discloses a fiber-optic gyro with a 3.times.3-coupler which operates close to-the so-called quadrature point, without requiring a modulator. Furthermore, U.S. Pat. No. 4,479,715 discloses a fiber-optic gyro with a modified 3.times.3-coupler which derives the rate of rotation from the quotient of the difference and the sum of two photo elements. The advantage of this signal analyzing process is that the intensity of the light source is cancelled out and its fluctuations therefore have no influence on the measuring signal. Furthermore, in this quotient formation, a variable coil damping is also cancelled out. However, a disadvantage of this simple quotient analysis is the limited usable range of the Sagnac phase at a value within .+-..pi./2, typically .+-..pi./3.
Furthermore, German Patent Document DE 39 12 005 C2, provides a signal analysis method for a fiber-optic gyro which compensates for contrast fluctuations by pseudo-statistical polarization fluctuations of the light in the fiber coil by computing processes. Also, in Applied Optics, Vol. 29, 1990, Pages 5360 and on, the rate of rotation for a fiber-optic gyro having a 3.times.3-coupler is computed by placing the sum or difference signal of the above-mentioned photo element signals in relation to the rate of rotation, to the contrast of the interference signal, to six different temperature-dependent coefficients as well as to the damping of the optical elements, particularly the fiber coil. The temperature-dependent coefficients are determined by calibration of the gyro, and depend only on the light source, the coupler (or couplers in the case of multi-axis gyros), and the photo elements, these values being easily reproducible in the case of recurring temperatures. The contrast depends on the characteristics of the coil and the polarization of the light. The damping of the light in the optical components, particularly in the fiber, is a function of temperature and is subject to an environmentally caused aging. In addition, in contrast to the other temperature-dependent coefficients, the damping has a comparatively large hysteresis in the case of changing temperature courses.
According to German Patent Document DE 39 12 005 C2, the contrast can be eliminated from the above-mentioned equations for the sum or difference signal. The advantage to the initially described quotient analyzing method is that the usable range of the Sagnac phase can be expanded to .+-..pi., thus to approximately the triple value. However, the determined rate of rotation still depends on the fiber damping. For this reason, it was attempted according to German Patent Document DE 39 19 060 C2 to calculate the value of the damping by means of a recursive algorithm. However, this is successful only if the gyro signals are known at the 0 rate of rotation and at a rate of rotation other than 0. The latter results in a significant operational limitation since, after switching-on the gyro, the 0 rate of rotation must exist at least once; in addition, the relatively long computing times required for the recursive algorithm impair the measuring speed of the gyro.
It is therefore an object of the present invention to provide a method for determining the rate of rotation of a fiber-optic gyro by means of which the influence of any damping variations on the measuring signals can at least be reduced.
This and other objects and advantages are achieved by the method according to the invention, which is based on the. consideration that, when stabilizing the contrast (for example, by using polarization-maintaining fibers according to German Patent Document DE 41 34 312, or using a fiber which is not polarization-maintaining, by means of an additional depolarizer or several depolarizers according to German Patent Document DE 41 20 675 and German Patent Document DE 39 42 819) the equations for the gyro signal analysis known from the above-mentioned Applied Optics document can be solved for the damping. The rate of rotation can then be determined by means of a damping value computed in this manner, together with the remaining measuring signals.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.